In the design and manufacture of running shoes, the emphasis has always been on providing a sole which absorbs the shock of running. In a shoe that is not custom-made, it has been necessary to compromise somewhat on the shape of the inner surface of the sole. For that reason, researchers have sought an ideal method of adjusting the shape of the inner surface of the sole to fit individuals. The obvious system involves providing an insole that can adjust itself to the shape of the individual foot. Therefore, the prior art discloses a number of insoles that are designed to be customized to an individual foot. For example, the U.S. Pat. No. 3,407,406 describes a shoe pad filled with microspheres which deform to fit the foot. The U.S. Pat. Nos. 2,794,270 and 3,903,621 disclose shoe linings containing semi-liquid moldable materials. However, constructions such as these cannot provide adequate support for a foot, since they continue to change shape under the pressures exerted by the wearer's foot during use. It is also well-known to inject liquid resin into a boot and to allow it to set while the wearer's foot is in position in the boot. This system is particularly used in connection with ski boots and, in theory, gives ideal support. In practice, however, such a liner is restricting, rather than supportive, and fails to allow for normal flexing of the foot during use.
It can be seen, then, that existing insoles tend to attenuate the forces associated with walking and running, whether they are incorporated in the shoe or are removable. The intensity of such forces is reduced, because of energy absorption by the material of which the insole is made.
Cellular materials are widely used in existing insoles, because they have actual (or are perceived to have) good energy absorption. Cellular materials, however, are characterized by poor dimensional stability, so that the end product may be flexible and wrinkle-free, but the necessary thickness of cellular materials is a disadvantage, since the shoe must be more voluminous in the vertical direction to accommodate such thick insoles. Furthermore, as such thickness increases, resistance to toppling ("support") decreases. Fabric surfaces are used widely in existing insoles, because they impart actual or perceived comfort, durability, color, or graphics. However, such fabrics are characterized by poor dimensional stability, so that the end product is undesirably flexible and is not wrinkle-free. Stiffness has never been desirable, because of discomfort and user fatigue. Wrinkling is to be avoided, also, because of discomfort and wear to the product as the pressure of the foot is concentrated during running on the lines of the wrinkles. Similarly, soiling is concentrated by the hills and valleys of the wrinkles.
In existing insoles, attenuation is limited to the volume of the insole affected by the associated forces. The rates of force transmission and the dimensions of the affected volume tend to be low. Accordingly, the decrease in intensity is not effective over a large area of the foot. The characteristics of the materials result in low transmission distances and velocities, these characteristics being notable in the low modulus of flexible cellular materials and the yarn geometry of fabric surfacing materials. Pigments tend to abrade off these materials when they flex during use. In the known insoles, the direction of force transmission is not controlled effectively. The rates and the quantities transmitted tend to be equal in every direction through the insoles and away from said forces. Thus, the forces are transfered through existing insoles to the foot in greater density near the site of said forces on the insole and the dispersal of said forces is poor.
Higher modulus materials are not used in existing insoles, because of their poor flexibility or tendency to crack at high loads, these all being negative attributes. Existing insoles have little puncture resistance, so that anything puncturing the outsole tends to puncture the insole also. Closed-cell cellular materials tend to rupture during use in existing insoles and to become open cells; this diminishes their attenuation capacity. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.
It is, therefore, an outstanding object of the invention to provide an insole that is especially fitted to the person who is to use the shoe.
Another object of this invention is the provision of an insole which is extremely thin, but which offers substantial support to the foot.
A further object of the present invention is the provision of an insole that offers greater sliding between the insole and the stocking than exists between the stocking and the foot, thus preventing the development of blisters.
A still further object of the invention is the provision of an insole that is simple and rugged in design, which can be readily manufactured, and which is capable of a long life of useful service.
It is a further object of the invention to provide an insole which has a high modulus of elasticity, but which will not develop cracks or form wrinkles.
Another object of the invention is the provision of an insole which is thin, which is supportive of the foot, which is comfortable, which is durable, which is capable of bearing permanent graphics, which is wrinkle-free, which is capable of dispersing forces rapidly, and which is broadly inclusive of some high-modulus material. The invention has the attributes of being foot-shock protective, foot-wear protective, crack free, and puncture resistant.
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.